The present invention relates to a method of and to an apparatus for turning (changing the orientation) of stacks of sheets and/or panels which are made of paper and/or other materials. More particularly, the invention relates to improvements in methods of and in apparatus for turning stacks of paper sheets or the like in such a way that two discrete parts of a stack to be turned rest on conveyors which define two elongated parallel paths and one of the conveyors is faster than the other conveyor. Conventional turning apparatus whose operation is based on the above principle are disclosed, for example, in German Offenlegungsschrift No. 32 21 601.
It is often necessary or desirable to turn stacks of paper sheets or the like through 90.degree., through 180.degree. or through another angle during transport from a preceding station to the next-following station in a production line wherein the stacks are formed and/or processed, e.g., in a production line for the making of stationery products in the form of exercise pads, accumulations of maps, charts and the like as well as during the making and/or treatment of pamphlets, brochures, books, booklets and similar products. As a rule, the stacks are transported in such positions that their sheets and/or panels are horizontal or nearly horizontal, i.e., it is necessary to turn the stacks about vertical or nearly vertical axes extending at right angles to the planes of the respective sheets. The aforementioned Offenlegungsschrift discloses a turning apparatus wherein two elongated parallel conveyors define two parallel paths and the stacks are deposited on the conveyors in such a way that a first part of the lowermost sheet or panel contacts one of the conveyors while another part of the lowermost sheet or panel contacts the other conveyor. The conveyors are driven at different speeds, and the difference between the two speeds will determine the extent to which the orientation of a stack is changed during travel along the two paths. It has been found that the just described conventional apparatus cannot ensure a predictable change of orientation of a long or even a short series of successive stacks. The reason is that (in addition to the difference between the speeds of the two conveyors) the extent to which a stack will be turned depends on a variety of parameters such as friction between the lowermost sheet of a stack and the adjacent conveyors, the area of contact between the lowermost sheet and the conveyors, the original orientation of a stack and others. Therefore, it is invariably necessary to correct the orientation of stacks which have completed their travel through a conventional turning mechanism. Accurate orientation of stacks which have advanced beyond a conventional turning apparatus is not only desirable but often critical, e.g., when the stacks are to be admitted into a trimming machine or into a packing machine. Prior proposals include the provision of mechanical orientation correcting means in the form of stops, guide rails, endless belts which travel along the path for a freshly oriented stack downstream of the turning apparatus, and others.